In recent years, α-olefins, particularly 1-hexene, 1-octene and the like are widely used as comonomers of linear low density polyethylene (L-LDPE), and α-olefins up to 1-tetradecene having 14 carbon atoms including the above α-olefins are widely used in an application of synthetic lubricants and as raw materials for surfactants. Usually, the above α-olefins are obtained by polymerizing ethylene, and a polymerization degree distribution of α-olefins obtained satisfies the relation of obtaining a dimer in the largest amount, a trimer in a larger amount than that of a tetramer and a n-mer in a larger amount than that of a (n+1)-mer in an ordinary distribution according to a Shultz-Flory distribution.
At present, α-olefins are obtained according to the above Shultz-Flory distribution in many processes in which α-olefins are industrially produced. For example, a process in which α-olefins are produced by using a triethylaluminum catalyst is known, and it is known as well that involved therein are the problems that the α-olefins are obtained according to the Shultz-Flory distribution and that in addition thereto, the catalyst has to be used under the condition of a high temperature and a high pressure. However, α-olefins are different in a consumption amount and a price depending on carbon numbers thereof and heavily fluctuated in needs therefor. Accordingly, conventional processes in which α-olefins are produced according to the Shultz-Flory distribution have involved the problem that α-olefins having carbon atoms which are not less than those of particularly 1-octene have low yields. As a countermeasure for the above problem, the triethylaluminum catalyst is used to carry out a two-stage polymerization, whereby provided is the advantage that α-olefins having carbon atoms which are not less than those of 1-octene are obtained in large amounts according to a Poisson distribution. However, the defects of having to carry out the reaction under a high temperature and a high pressure using a large amount (stoichiometric amount) of the catalyst, indispensable recovering of the catalyst, complication of the apparatus and a reduction in a purity of the α-olefins produced have been involved therein. For example, processes in which 1-hexene and 1-octene are produced by using chromium compounds are disclosed in patent documents 1 to 4. However, it is reported therein that α-olefins obtained by the above processes comprise 1-hexene as a principal component and that a yield of 1-octene is low. On the other hand, transition metal compounds comprising aminophosphine as a ligand are disclosed in a patent document 5, but an object thereof is to produce polymers.
In light of the backgrounds described above, catalysts for enhancing the yields of α-olefins of 1-hexene up to 1-tetradecene each having 6 to 14 carbon atoms, particularly a yield of 1-octene are desired to be developed.